Enriching Scala collections with a method

Question

How do I add a foreachWithIndex method on Scala collections?

This is what I could come up with so far:

implicit def iforeach[A, CC <: TraversableLike[A, CC]](coll: CC) = new {
  def foreachWithIndex[B](f: (A, Int) => B): Unit = {
    var i = 0
    for (c <- coll) {
      f(c, i)
      i += 1
    }
  }
}

This doesn't work:

Vector(9, 11, 34).foreachWithIndex { (el, i) =>
  println(el, i)
}

Raises the following error:

error: value foreachWithIndex is not a member of scala.collection.immutable.Vector[Int]
Vector(9, 11, 34).foreachWithIndex { (el, i) =>

However the code works when I explicitly apply the conversion method:

iforeach[Int, Vector[Int]](Vector(9, 11, 34)).foreachWithIndex { (el, i) =>
  println(el, i)
}

Output:

(9,0)
(11,1)
(34,2)

How do I make it make it work without explicit application of a conversion method? Thanks.

Answer 1

You need to extend Iterable:

class RichIter[A, C](coll: C)(implicit i2ri: C => Iterable[A]) {
    def foreachWithIndex[B](f: (A, Int) => B): Unit = {
    var i = 0
    for (c <- coll) {
      f(c, i)
      i += 1
    }
  }
}

implicit def iter2RichIter[A, C[A]](ca: C[A])(
    implicit i2ri: C[A] => Iterable[A]
): RichIter[A, C[A]] = new RichIter[A, C[A]](ca)(i2ri)

Vector(9, 11, 34) foreachWithIndex {
  (el, i) => println(el, i)
}

output:

(9,0)
(11,1)
(34,2)

See this post by Rex Kerr for more information.

Answer 2

The short answer is that you have to parameterize CC if you do it that way or the type inferencer can't figure out what A is. The other short answer is do it the way I describe in the answer to this question.

To expand a little bit more, there's really no reason that you need CC <: TraversableLike--just have it take a Traversable and start with iforeach[A](coll: Traversable[A])! You don't need to use fancy type bounds in order to use a superclass/supertrait. If you want to do something more complicated where you return another collection with collection type preserved, then you need to use builders and such, which I describe in the other question.